1. Field of the Invention
This invention relates to a security seal. Such seals are used to maintain the security of goods in transit, in that any attempt to open the seal by unauthorised personnel should leave visible evidence of tampering.
2. Description of the Prior Art
The invention is particularly concerned with seals of the kind known as strip seals. These comprise a flexible strip, one end of which is fixed inside a locking box or chamber. In use, the free end of the strip is bent back and inserted through an entry slot in the locking box for non-detachable locking therein. Strip seals can be sub-divided into two main categories: ball strip seals and flat strip seals, both of these terms referring to the shape of the locking box.
In ball strip seals, the portion of the first end of the strip inside the ball-shaped locking box is bent back upon itself to form a channel for receiving the free end of the strip. In the locked position, holes in this channel section and in the free end of the strip are in register and locking is effected by one or more, usually two, spring steel circlips(circular clips) which pass through the holes. The circlips have free ends which, in the unlocked position, are resiliently spaced apart by the outer faces of the channel section backward (with reference to the direction of insertion of the free end of the strip) of the holes in the channel. As the free end of the strip is inserted, the leading edge thereof urges the circlips forward until, when all the holes are in register, the free ends of the circlips spring into the holes to effect locking. The dimensions of the holes are chosen so that there is a certain degree of free play of the free end of the strip in the locked position. This feature is utilized to provide visible evidence that the seal is locked. Opposite the entry slot in the locking box there is an exit slot and, in the locked position, it is possible to push the free end of the strip forward so that it partly protrudes from the exit slot, showing that the seal is locked. Once the seal is locked as described above, it is no longer possible to open it without breaking open the locking box or cutting the flexible strip, and in both cases there will be clear evidence of tampering. Usually, the strip is marked with identification letters and/or numbers to make each seal individually identifiable.
In flat strip seals, the end of the strip fixed inside the locking box is bent back upon itself and formed into a catch (rather like a fish hook). The free end of the strip has a hole in it so that, when it is inserted into the locking box, it passes over the catch and the latter springs back at an angle at 45.degree.-60.degree. to engage with the hole. Subsequent withdrawal of the end of the strip is thereby impeded.
In operational conditions, particularly outside in adverse weather conditions, the method described above for checking that a ball seal is locked, i.e. causing the free end of the strip to protrude from the locking box, can be difficult to carry out reliably in practice. Another recommended test to ensure locking, and which applies both to ball seals and to flat seals, is the so-called "tug" test which is to circumvent anyone merely placing the free end of the strip into the locking box to give the appearance that the seal is locked. In this test, the strength of the circlip mechanism in ball seals is far superior to that of the catch mechanism in flat seals.
A serious weakness both of ball seals and of flat seals is the vulnerability of the locking box itself. The locking box in ball seals usually comprises a ball housing of two substantially hemispherical parts that are swaged together creating a seam of varying strength of vulnerability. In some designs, a separate additional piece of material is crimped over the overlapping portions. Various illicit methods of opening the locking box and then re-sealing it have been developed. For example, the box may be cut open with a very fine saw, thereby giving access to the interior so that the circlips can be opened. The mechanism can then be re-set and the locking box re-sealed with glue. The end result can be very difficult to detect, especially under operational conditions.
An object of the present invention is to overcome the problems outlined above in connection with ball seals, while preferably maintaining the superior strength of the ball seal in comparison with flat seals. In particular, we have sought to provide an improved means for indicating that the seal is in the locked position, and to provide improved security for the locking box itself. Although the invention is particularly useful in connection with ball strip seals, it also has general applicability to strip seals.